Project Summary Apoptosis is a universal feature of normal development and aging. This naturally occurring form of cell death is firmly established in the etiology, pathogenesis and treatment of many human diseases. Our research seeks a comprehensive understanding of molecular networks that support apoptotic cell death using the Drosophila model system. In the previous grant period we explored in vivo properties of the apoptosome, a pivotal molecular complex that lies at the heart of apoptotic pathways throughout the animal kingdom. Our analyses of apoptogenic mutants revealed conspicuous phenotypes and uncovered a form of collective cell death, where waves of apoptosis prompt the sudden elimination of epithelial tissue with features that resemble glandular involution and ischemic pathologies. We initiated a genetic dissection of this process and, in complementary efforts, we also completed a genome-wide screen for new apoptotic effectors. From these in vivo and ex vivo approaches, we captured novel gene sets that are obligate for cell death. Our first aim leverages unique opportunities in this tractable model to investigate implicated factors and communal properties that govern collective apoptosis. Aims 2 and 3 examine highly conserved genes not previously implicated in cell death. Aim 2 starts with allelic mutations, arrested at an early step in collective apoptosis, and determines how the corresponding gene acts to control cell death. Aim 3 starts with an effector required for caspase activation, advancing a comprehensive in vivo analysis of apoptotic functions specified by this gene product. These combined projects will advance general principles and novel determinants that regulate cell death in vivo. Because molecular pathways governing cell death are conserved, insights resulting from these efforts could illuminate new rationales for the treatment of neoplastic or degenerative pathologies, where misregulated apoptosis is a root cause.